A carbon nanotube possesses excellent physical and mechanical properties. The microstructure of a carbon nanotube may be viewed as a seamless hollow tube formed by rolling a graphite sheet. It has a very large aspect ratio, generally with a diameter in the range of 1-100 nm, and a length of several microns to over one hundred microns. A carbon nanotube shows superior dynamic and electric properties. It has a hardness comparable with that of diamond, and a Yang's modulus of about 1.8 TPa. Its tensile strength is about 200 GPa, 100 times higher than the strength of a steel, but its weight is only ⅙ to 1/7 of the weight of the latter. Meanwhile, the maximum elastic strain of a carbon nanotube is up to about 12%, and thus it is as flexible as a spring. A carbon nanotube has an electric conductivity up to ten thousand times that of copper, and its heat conductivity is very good as well. Owing to their superb properties, carbon nanotubes are expected to be used widely in a variety of fields such as nanoelectronic devices, catalyst supports, electrochemical materials, composite materials and the like.
Realization of production, particularly mass production of carbon nanotubes is a prerequisite for application of carbon nanotube technology, and it's also a bottle neck constraining industrialization of this technology. Low carbon alkanes, alkenes, alcohols and the like are the main carbon sources for preparation of carbon nanotubes, but larger molecular weight carbon sources such as cyclohexane, benzene, phenanthrene and the like can also be used. Low carbon sources favors splitting, but the cost is relatively high. Due to increasing market competition, more and more efforts have been devoted to studies on low cost heavy carbon sources. Zhang Jun, et al synthesized carbon nanotubes from crude paraffin as a carbon source using an explosion process, and tried coal tar and asphalt which were used as carbon sources to prepare carbon nanotubes [Coal Conversion, 33(1), 2010], but the resulting carbon nanotubes had a low purity of about 70%. Despite low cost, the use of heavy carbon sources generally requires a relatively complex preparation process with low purity carbon nanotubes produced.
Nowadays, a fluidized bed reactor is typically used in an apparatus for preparation of carbon nanotubes, wherein a carbon source is split to form carbon nanotubes in the presence of a catalyst. However, a traditional gas-solid fluidized bed has the following disadvantages: reaction efficiency is affected significantly by the density and particle size of a catalyst; if the density of the catalyst is too low, the density of the product thus obtained is also relatively low, and thus the product may be blown out of the reactor in a short time, leading to decreased utility of the catalyst due to the insufficient retention time. In addition, if a low gas velocity is chosen for extending the retention time, the carbon nanotube product tends to form agglomerates which may clog the reactor at the upper part of the reactor, easily resulting in difficulty in fluidization in the course of production, among others. A variety of methods have been tried in the prior art to modify a traditional fluidized bed for preparation of carbon nanotubes.
For example, Chinese Patent Application CN1327943A discloses a method for continuous preparation of carbon nanotubes on a fluidized bed, wherein carbon nanotubes were grown on a catalyst support via chemical vapor deposition in the fluidized bed while the fluidization state of the generated nanocarbon material under the influence of a gas stream was controlled. Though this method solved the fluidization problem in the system to some extent, the utility of the catalyst was still low. Additionally, unreacted raw material gas and N2 were discharged directly through an exhaust system, leading to waste of raw material and pollution to environment, among others.
According to Chinese Patent CN 202519030U, a vacuum chamber and a collecting chamber were added to a fluidized reactor, wherein gaseous material that was not fully reacted was drawn out from the vacuum chamber after raw material reacted in a synthesis chamber and the resultants were moved into the vacuum chamber, and finally carbon nanotubes thus produced were collected in the collecting chamber. Albeit carbon nanotubes could be collected and isolated easily, the gaseous material drawn away was not recycled, so the utility of the raw material was low.
According to Chinese Patent CN 101475159B, a rotatory member was added to a fluidized bed reactor, wherein the rotator member included blades which rotated to prevent accumulation of a catalyst and reinforce fluidization, so as to increase reaction efficiency. This apparatus had a complicated structure and could not operate continually, unsuitable for scaled production.
According to Chinese Patent Application CN 101959793A, a support body was added into the inside of a fluidized bed, wherein gas paths of a certain width existed inside and around the support body, and a catalyst was loaded on the support body, so that fluidization was reinforced. This manner could provide relatively pure carbon tubes, but the reaction could not proceed continuously, and the efficiency was rather low.
According to Chinese Patent Application CN 102120570A, continuous production of carbon nanotubes was realized with the use of a series of reactors in tandem, but resulting in carbon nanotubes with a low purity of only about 90%.
According to Chinese Patent CN 100393616C, continuous production of carbon tubes was realized with the use of a tubular reactor. But the reactor was expensive, and the reaction state was difficult to control. So, it's not easy to be industrialized.
Despite all the improvements made to a traditional fluidized reactor, most of the technical solutions disclosed by the above references have disadvantages of low catalytic efficiency, low utility of raw material and inability of continuous operation, while the apparatus capable of continuous operation tend to be expensive, and produce a carbon nanotube product with low purity, frustrating realization of industrial production.
Therefore, there is still a need in the art to develop an apparatus capable of continuous preparation of carbon nanotubes, which shall has the advantages of simple structure, low cost, and ability to increase utility of catalyst and raw material. When this apparatus is used to prepare carbon nanotubes continuously, the resulting carbon nanotubes have the advantages of high purity and lower cost than those obtained by other methods. The product thus obtained has consistent quality, suitable to be produced industrially in large scale.